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The metabolism of our cities

This article discusses the main factors behind the growth of our cities and how energy has driven the growth of and layout of our built environment.


I discussed in a previous post that Sydney's main sources of energy are coal, oil and natural gas (92% of Australian energy consumption at 2021, with other sources being little more than a rounding error).


Before we had sources of energy such as this, ancient cities were MUCH more constrained in terms of size and population.


For example, it is estimated that the size of the city of Ancient Rome (the world's largest city at the time) reached as high as one million people. This would equate to an astounding population density of 72,150 people per square kilometre. (Sydney's currently highest density suburb of Pyrmont is 13,930 persons per square kilometre). High density was necessary with walking being the main mode of transport at the time.


Before 1800, there were very few cities that reached a population of more than 1 million and the world's population was far less urbanised. Depending on the source, possibly Alexandria, Baghdad and capitals of Ancient China (Xian, Beijing) may also have reached a population of 1 million, with most cities being well below that size.


People in Ancient times were more limited to energy from the sun hitting the earth at any given year to provide photosynthesis, the flow of water and wind, with some degree of stored energy in the form of cutting down trees and burning them, or their use as a construction material. It's therefore unsurprising that 8000 years ago, Europe was mostly a wooded continent, with today it covering less than half of Europe's land area.


With Great Britain the first country to switch over from these less dense forms of energy and to introduce coal as a significant source of it's energy mix, the city of London grew rapidly, reaching over 1,000,000 around 1800 and an extraordinary growth to 6,506,889, at the 1901 census.


Based on 2018 population estimates, there are now eight cities with an extraordinary population above 20,000,000 people (Tokyo, Dehli, Shanghai, Sao Paulo, Mexico City, Cairo, Mumbai, Karachi, Lagos, Moscow, Jakarta and Seoul).


Coal is formed as plants died, they sank to the bottom of swamps. Over the years, thick layers of plants were covered by dirt and water. They were packed down by the weight. After a long time, the heat and pressure changed the plants into coal. Oil and gas are a product of decomposed organic matter, typically from ancient marine microorganisms. Therefore both are subjects of photosynthesis from ancient sunlight that hit the earth millions of years ago. They are energy dense, as they represent compressed energy from ancient sunlight.


Today we take these ancient fossils, which are far more energy-dense than the energy sources used in Ancient Rome, extract them and burn them in machines such as below:


Truck (equivalent power 450-525 horses):



Modern SUV (around 245 horses equivalent):


(Doesn't seem like much, although there were 20.1 million registered motor vehicles in Australia in 2021).



Or this (Bayswater Power Station): 3,560,000 horses equivalent:


(This is an extraordinary number of horses, one can only imagine the amount of land that would be required to feed/sustain them).



The energy from these ancient fossil sources is being shown in the image below of Sydney at night, whereby the energy in light and heat is returning to outer space - where it came from millions of years ago.



Surplus food would be the other factor behind exponential city growth we have seen over the past 200 years, with an estimated 2.8 fossil fuel calories in every calorie of food that we eat. In some respects, petrochemical fertilisers have reduced pressure on deforestation and loss of habitat that would otherwise be the case with our expanding population, as less equivalent land has been required to grow the same amount of food compared to ancient times.


Certain parts of the city have a higher energy metabolism than others.


For example, the below image shows an aerial photo of the suburb of Glebe in inner Sydney. This suburb is characterised by:


Source: Google Maps


- High density, with most dwellings sharing common walls and/or roofs

- Smaller areas of road per person

- Smaller overall dwellings requiring less energy for heating (and cooling)


The image below shows the suburb of Grasmere, on the outskirts of Sydney (Camden). This suburb was developed more recently and is characterised by:


Source: Google Maps


- Large areas of public road per dwelling

- Large distances of public utilities per person (water pipelines, electricity cables, stormwater pipe etc.)

- Large houses per person with no areas of common wall, roof etc.

- Large areas of grass maintained by mechanical lawnmowers and used for few practical purposes.

- High dependence on private motor vehicles for transport and little or no public transport.


All of the above requires higher amounts of energy to maintain a residential lifestyle in this location (both for the individual occupants of these dwellings and for society/government/utilities as a whole). It is therefore very dependent on high consumption of oil-based energy products at affordable prices, as the most productive energy source we have ever discovered, whereas the layout of Glebe was developed before oil became a major part of the energy mix.


It's estimated that we're burning fossil fuels at over 4000 times the rate of 1776 and around 1,000,000 times the rate at which they were/are being naturally formed in the earth's crust.


Some governments have sought to address the unsustainability of the above occurring indefinitely by seeking to go back to the sources of energy that were predominant before 1800. For example the UK (original source of the industrial/energy revolution) government has provided subsidies to the owner of a major coal plant to switch to burning wood pellets. As most of England's forests have already been cut down over the past 1000 years, much of this timber is imported from other continents using oil powered machinery.


There will be much pressure on the earth's remaining forests and natural habitat, if more governments follow this path back to less dense forms of energy, as we no longer have the population of less than 1 billion people that we had in 1800, but today it is 8.1 billion.


The Ancient Romans and others demonstrated that human civilisation and even large cities were possible in the pre-fossil fuel era, although nothing like the ones we have today. The higher metabolism parts of the city would likely be the first areas of the city that would need to adapt under a lower energy scenario.

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